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Male shortage When males are thin on the ground, pairs of female albatrosses can work together to raise the next generation by themselves, Hawaiian researchers have found.

The findings, reported this week in the Proceedings of the Royal Society B, have emerged from an unusual colony of Laysan albatrosses that live on the island of Oahu, where females significantly outnumber males.

A decade ago researchers found that 31 per cent of breeding pairs in the colony consisted of two unrelated females who had formed partnerships, some lasting years, to raise young.

"We genetically sexed birds in this colony … and realised that what we had assumed were a male and a female were in fact two females," says Lindsay Young, a wildlife biologist from independent research organisation Pacific Rim Conservation.

Laysan albatrosses can only lay and incubate one egg each year and will not lay another if that year's egg is lost. In the Oahu colony, male birds that have a regular partner will sometimes also mate with another female, says Young.

"Basically, the males are cheating on their regular partners and this is how the eggs of two females are fertilised. The two females then pair together to cooperate in rearing the chick."

Best of a bad job

In their latest paper, Young and her colleagues explored this situation in more depth, monitoring 145 female Laysan albatrosses in the colony from 2003 to 2012 as they incubated eggs and fed their offspring.

On average, pairs of females successfully raised an average of one chick every four years, less than half as many per year as the male-female pairs, which raised an average of two chicks every three years.

The researchers also found that birds in female-female pairs lived shorter lives. This is probably because one was forced to incubate the eggs immediately after laying, instead of going to sea to feed.

"Normally, males take the first long incubation shift of three weeks while the female goes to feed after laying an egg, and after that they alternate every three to seven days," says Young.

Although the female-female pairs were less successful overall, their partnerships were better for the colony than not reproducing at all, the researchers note.

So the take home message is that for these albatrosses same-sex pairing can be a beneficial and evolutionarily adaptive strategy, Young and colleagues write.

"Compared with the option of not breeding at all, female-female pairing may indeed be 'making the best of a bad job' in response to a shortage of males."

'Choosy' males

Intriguingly, the researchers also found that females who successfully reared a chick with another female were more likely to find a male partner in following seasons.

"Males as well as females were capitalising on the female shortage by being 'choosy' and only pairing with the females who had the highest chance of raising an offspring," says Young.

"Typically we think of females as being the choosy sex and selecting for male ornamentation (think peacocks)," she says. "But in this case the males are being choosy by picking the best mothers for their offspring."